The invention relates to a process for the production of liquid pig iron or liquid steel pre-products from charging substances comprising iron ore and fluxes and at least partially containing a portion of fines, wherein the iron ore is directly reduced to sponge iron in at least two reduction stages by the fluidized bed method, the sponge iron is melted in a melt-down gasifying zone under the supply of carbon carriers and an oxygen-containing gas, and a CO- and H2-containing reducing gas is produced which is injected into reduction zones of the reduction stages, is reacted there, is withdrawn as a top gas and optionally is supplied to a consumer, and a plant for carrying out the process.
A process for the reduction of ore with subsequent melting is known for example from EP-A-0 594 557. With this known process, in accordance with a preferred embodiment, reduction is carried out in two locally separated reduction zones connected in series, wherein the reducing gas exiting the first reduction zone is supplied to the second reduction zone, which is connected to precede the first reduction zone in the direction of flow of the fine ore, hence in counter-flow, and from there under compression is supplied to a preheating zone. Each of the two reduction zones has an upper section in which fine solid particles are reduced in a fluidized bed and a lower section to which coarser solid particles descend and in which they are reduced in a flown-through fixed bed.
Hereby, advantages result as compared to single-stage direct reduction, i.e. to direct reduction utilizing only a single reduction zone, said advantages consisting above all in a low consumption of reducing gas, namely for the following reason: technical reduction processes require a reduction temperature of at least 750xc2x0 C., so that there inevitably results a minimum temperature of the reducing gasxe2x80x94when exiting the reduction zonexe2x80x94of 750xc2x0.
Since for technical reasons it is not admissible for the reducing gas from the melter gasifier to have temperatures in excess of 950xc2x0 C., only a temperature gradient of roughly 200xc2x0 C. is available, meaning that only roughly ⅓ of the sensible heat of the reducing gas can be utilized. To be able to maintain the above-indicated temperature level, it would be necessary with a single-stage reduction process to utilize reducing gas in an amount several times the amount required for reduction. This would result in insufficient exploitation of the reducing gas and hence in a high level of coal consumption in the melter gasifier.
Although this known process has proved its value, different degrees of reduction may result with the fine-grain fraction and the coarse-grain fraction of the iron ore when processing ores of different grain sizes, that is, when processing ores having a slightly higher portion of fine ore (e.g. run-of-mine ore). Remediation is difficult, as it is not possible with this known process to adjust the retention time of the fine-grain fraction independently of the retention time of the coarse-grain fraction of the iron ore in the reactor vessels.
With the known process, the completely reduced fine ore portion from the reduction zone arranged to immediately precede the melt-down gasifying zone is charged to the melt-down gasifying zone separately from the coarse ore portion, namely at the height of the fluidized bed forming above the fixed bed of the melt-down gasifying zone. Hereby, conveying-out of the fine-grain fraction along with the reducing gas generated in the melt-down gasifying zone is avoided. If the fluidized bed becomes overloaded with the charged fine-grain fraction, breakdown of the fluidized bed and subsequently damming-up of gas may ensue. This results in eruptive outbreaks of gas. Hereby, the gasification process for the carbon carriers and the melt-down process for the reduced iron ore, that is the sponge iron, is markedly disturbed. Uncontrollable fluctuations in the pressure and quantity of the generated reducing gas and formation of a reducing gas having a reductant composition which is disadvantageous to the reduction process may ensue.
From KR patent application 94-38980, a process of the initially described kind is known in which in the reduction zone arranged to imnmediately precede the melt-down gasifying zone the prereduced fine ore portion is discharged by means of the reducing gas and supplied to a separate fine ore reduction zone. From the latter, the completely reduced fine ore is also conducted to the fluidized bed zone in the melter gasifier, as according to EP-A - 0 594 557, so that, here, the disturbances already described above may occur in the melter gasifier.
In accordance with KR patent application 94-38980, the ore is prereduced in a first reduction zone, with the fine ore portion and the coarse ore portion being reduced together in a single reduction zone. This results in the disadvantages described in connection with EP-A - 0 594 557, namely in nonuniform degrees of reduction of the fine ore portion and of the coarse ore portion in this reduction zone.
The invention aims at avoiding these disadvantages and difficulties and has as its object to provide a process of the initially described kind as well as a plant for carrying out the process, by which not only a uniform reduction of the fine portion and coarse portion of the ore is feasible, namely in a reduction process which, in order to achieve good gas exploitation of the reducing gas, is a multiple-stage, i.e. at least two-stage, reduction process. In particular, disturbances of the melt-down process and of the production process for the reducing gas in the melt-down gasifying zone are also to be avoided herein.
With a process of the initially described kind, this object is achieved in accordance with the invention in that:
each of the two reduction stages is provided with two separate fluidized beds, wherein in a first reduction stage the iron ore by aid of the reducing gas is fractionated into at least two fractions having different grain size distributions each, namely into at least one coarse-grain fraction and at least one fine-grain fraction,
each fraction is reduced by the reducing gas in a separate fluidized bed, wherein
the reducing gas maintains a first fluidized bed containing the coarse-grain fraction and separates the fine-grain faction from the same,
and wherein, further, reducing gas is additionally introduced into the further fluidized bed directly, in an amount and/or chemical composition such that reduction of the fine-grain fraction in this fluidized bed to a predetermined degree of metallization within a predetermined period of time is ensured, and
reduced iron ore is discharged both from the first and from the further fluidized bed and
the fine- and the coarse-grain fraction reduced in the first reduction stage are further reduced in a further reduction stage operating in the same manner as the first reduction stage and from the last reduction stage the fine-grain fraction is introduced into the melt-down gasifying zone while being agglomerated by provision of oxygen, preferably by means of a burner, and the coarse-grain fraction is fed directly into the melt-down gasifying zone gravitationally.
The charging of a reduced fine-grain fraction to a melt-down vessel by means of a burner is known per se from KR patent application 92-27502. But here, reduction by the reducing gas is effected in a single stage and melting down of the ore that is only prereduced in the single-stage process takes place by the so-called xe2x80x9cin-bathxe2x80x9d method. In accordance with this method, only a metal melt covered by a molten slag, without a fixed bed and without a fluidized bed, is present in a reactor vessel. The charged coal gasifies in the slag layer in which the charged prereduced ore is also completely reduced. However, the reduction process takes a completely different course than with the process of the initially described kind and the process in accordance with the invention, as, in prereduction, reduction of Fe2O3 by means of CO and/or H2 is, at the most, only carried to the FeO stage and the prereduced ore is then completely reduced in the melt-down vessel by means of carbon, namely in accordance with the equation FeO+C=Fe+CO. These xe2x80x9cin-bathxe2x80x9d melting processes are therefore fundamentally different from the process of the initially described kind, because reduction by a reducing gas is effected only to a slight extent, namely to a degree of reduction of roughly 30%. For complete reduction in the melt-down reactor, a high percentage of carbon is required if compared to the process according to the invention, whereas with the process of the initially described kind and in accordance with the invention reduction to a degree of reduction of 90% or more is carried out exclusively by reducing gas. Since with the xe2x80x9cin-bathxe2x80x9d method there is no fixed bed and no fluidized bed, the problem underlying the invention, i.e. overloading of the fluidized bed, does not occur.
In accordance with a preferred embodiment, in both reduction stages the grain size distribution of the separated fine-grain fraction according to the invention is adjusted as a function of the overall grain size distribution by adjusting the amount of reducing gas supplied to the first fluidized bed per time unit and, at the same time, the degree of reduction of the fine-grain fraction is adjusted by adjusting the amount of secondary reducing gas which is directly supplied to this fraction additionally.
A simplified embodiment of the process according to the invention provides that the fine- and the coarse-grain fraction reduced in the first reduction stage are further reduced in the first fluidized bed of the further reduction stage together and the fine-grain fraction is once again separated and supplied to the further fluidized bed and there is further reduced.
Suitably, the fine-grain fraction reduced in the first reduction stage is supplied to the further fluidized bed of the further reduction stage directly and is further reduced there.
Another simplified process variant of the process set forth in the invention is characterized in that instead of via a burner the fine-grain fraction is introduced into the melt-down gasifying zone in close proximity to an oxygen feeding means opening into the melt-down gasifying zone.
A plant for carrying out the process according to the invention comprising at least two reduction units arranged in series, from which there run into a first reactor vessel a conveying duct for charging substances containing iron ore and fluxes, a gas feed duct for a reducing gas and a conveying duct destined for the reduction product formed in said reactor vessel and leading to a further reduction unit with a reactor vessel, and a gas discharge duct for the top gas, wherein the gas feed duct for the reducing gas forms a gas discharge duct for reducing gas from the further reduction unit and a further conveying duct for the reduction product formed in the further reduction unit runs into a melter gasifier provided with supply ducts for oxygen-containing gases and carbon carriers as well as with a tap for pig iron or steel prematerial and slag, wherein the reducing-gas feed duct for reducing gas formed in the melter gasifier which runs into the further reduction unit departs from the melter gasifier, is characterized in that each of the reduction units is provided with at least two reactor vessels arranged in series in the direction of flow of the iron ore, each reactor vessel having a separate fluidized bed therein, and with one gas feed duct for the reducing gas leading to each of said reactor vessels in parallel arrangement, wherein from the reactor vessel that is first if viewed in the direction of flow of the iron ore a reducing-gas discharging means runs into the second reactor vessel of the same reduction unit intended for the fine-grain fraction of the iron ore to be reduced and a conveying duct for the reduction product departs from each reactor vessel, and wherein further the two conveying ducts leading out of the first reduction unit run into the further reduction unit and the conveying ducts departing from the further reduction unitxe2x80x94in case this forms the last reduction unitxe2x80x94lead to the melter gasifler separately, namely a conveying duct departing from the first reactor vessel of the last reduction unit enters the melter gasifier directly and a conveying duct departing from the second reactor vessel of the last reduction unit enters the melter gasifler at an oxygen-enriched site, preferably via a burner.
According to a preferred embodiment, the two conveying ducts leading out of the first reduction unit enter the further reduction unit together.
Suitably, the conveying duct leading out of the further reactor vessel of a reduction unit runs into the further reactor vessel of the subsequently arranged reduction unit directly.
Another preferred embodiment is characterized in that the first reduction unit is preceded by a pre-heating vessel for the iron ore into which there enters a gas duct conducting a top gas from the first reduction unit.